In February 2015, Robin Thicke took his show to a federal courtroom in Los Angeles. Testifying in the lawsuit over his 2013 hit “Blurred Lines,” Thicke played and crooned a piano medley of U2’s “With Or Without You,” the Beatles’ “Let It Be,” Alphaville’s “Forever Young,” Bob Marley’s “No Woman No Cry,” and Michael Jackson’s “Man in the Mirror.” The unusual live-from-the-witness-stand performance was meant to show that “Blurred Lines” didn’t infringe on the copyright of Marvin Gaye’s “Got to Give It Up,” because many songs can share similar chords or melodies without being copies. A jury disagreed: Pending an appeal, Thicke and collaborators Pharrell Williams and T.I. owe Gaye’s family about $5.3 million.

On June 14, after some delay, Led Zeppelin’s Jimmy Page and Robert Plant are themselves due to appear in an L.A. federal courtroom. They’ll be battling claims that “Stairway to Heaven,” the band’s 1971 classic-rock staple, infringes on the copyright of “Taurus,” an instrumental released in 1968 by the less-famous group Spirit. The legal outcome is as uncertain as the whims of a jury. Whatever happens, though, the trial puts a focus on the gymnastic arbitrariness of copyright law in the digital age. Now that YouTube and social media make it easy for the public to see how almost everything sounds like something else, at what point do we decide the similarities are inappropriate? And with smaller revenue streams for musicians overall, when do they deserve to get paid for those overlaps?

Debate over the resemblance between “Stairway” and “Taurus” wasn’t new when a trustee for the estate of Spirit guitarist Randy California (né Wolfe) filed suit in May 2014. Before Wolfe died in 1997, he discussed the similarity in an interview with Listener magazine, according to the lawsuit. Trustee Michael Skidmore filed the lawsuit in between a Supreme Court decision on long-delayed copyright infringement cases and a remastered re-release of “Stairway.” Listening to the songs side by side, it’s not hard to imagine how Page might’ve heard something in the older song that helped inspire him to create something far grander and more widely resonant. U.S. District Court Judge Gary Klausner acknowledged as much in his ruling that the case could go to trial, writing that “the similarities here transcend” the shared use of standard musical tropes.

At the same time, the lines between what’s a commonplace songwriting convention and what’s a transcendent ripoff have become increasingly, well, blurred. Tom Petty gained a songwriting credit on Sam Smith’s 2014 song “Stay With Me” nearly a year after its release, though as The Guardian points out, the kinship between Smith’s track and Petty’s 1989 hit “Won’t Back Down” has little to do with what’s memorable about either song. Complicating matters further, Petty once gave an interview shrugging off the similarity between the ringing guitars on the Strokes’ 2003 hit “Last Nite” and his 1976 song “American Girl.” Meanwhile, Beyoncé’s extensive credits for her new albumLemonade include Animal Collective because, her producer Boots has said, of an “accidental” borrowing involving the phrase “material things.” Elsewhere, Justin Bieber and Skrillex are fighting claims their hit “Sorry” copies a vocal riff from a song by independent artist White Hinterland. And in May 2015, without fanfare, Mark Ronson’s “Uptown Funk” gained five additional authors, almost six months after its release. When a similarity warrants sharing the songwriting credit and when it’s no big deal is becoming tough to discern.

Dylan’s argument—that certain styles of music have a “tradition” of borrowing—applies to newer musical genres, too. When Drakefaced criticism for “Hotline Bling” sounding like D.R.A.M.’s months-older “Cha Cha,” he brought up the form of copying customary in Jamaican dancehall music. “You know, like in Jamaica, you’ll have a riddim and it’s like, everyone has to do a song on that,” Drake told an interviewer. “Imagine that in rap, or imagine that in R&B.” It’s a classic case: The wealthy pop star asking us to imagine no possessions. But he isn’t wrong about a world of musical inspiration that’s ripe for creative people in various traditions to share.

The blues, of course, has its own vital legacy of musical interpolation. Zeppelin’s history suggests this tradition of borrowing gets a whole lot messier when the borrower enjoys commercial success without crediting the borrow-ee. Also long after the fact, the band added Chicago blues legend Willie Dixon to the credits of 1969’s “Whole Lotta Love” (for his “You Need Love,” first recorded by Muddy Waters in 1962) and folk singer Anne Bredon to 1969’s “Babe I’m Gonna Leave You” (Zeppelin covered Joan Baez’s 1962 version, which was thought to be a traditional song, until the Zep version came to Bredon’s attention). “Dazed and Confused,” also released in 1969, is now credited as being written by Page and “inspired by Jake Holmes,” the New York folk-rock singer-songwriter who recorded his own “Dazed and Confused” two years earlier. Clearly, Page and Plant are capable of sharing credit where it’s due—or at least when pressured.

The jury in the “Stairway” case won’t be allowed to hear about Zeppelin’s history of awarding songwriting credits long after the fact, the judge has ruled. Also, as in the “Blurred Lines” case, the jury will have to decide based on the written versions of the songs, rather than the recordings. Perhaps Page and Plant, who have denied hearing “Taurus” and claim they came up with “Stairway” autonomously, will regale the court with their own guitar-and-voice medley. The risk for them is that legal standards are different now: Music industry lawyer Jay Rosenthal recently told the New York Times that where in the past these lawsuits centered on melody and lyrics, the “Blurred Lines” verdict has made rhythm and “feel” fair game, as well. As the lawyer who filed suit against Zeppelin, Francis Alexander Malofiy, told me in April, “I don’t believe a jury will be as forgiving as Led Zeppelin's fans.”

Whatever results from the latest case, the debate over what’s theft in songwriting and what’s affectionate borrowing doesn’t look likely to end anytime soon. Is it too late for Tom Petty to say sorry?